GRANITE POWDER AS A SOURCE OF POTASSIUM FOR PLANTS - A GLASSHOUSE BIOASSAY COMPARING 2 PASTURE SPECIES

The effect of granite powder (<70 mu m) as a K fertilizer was investig ated in a glasshouse pot experiment conducted with three acid, sandy t opsoils from podzols of South Western Australia and with three fertili zer treatments: a control without K application, a KCl treatment (90 m g K kg(-1) soil) and a granite treatment (20 g granite kg(-1) soil, yi elding 640 mg K kg(-1) soil). Subterranean clover (Trifolium subterran eum) and ryegrass (Lolium rigidum) were cropped in triplicated pots fo r 7 weeks, harvested and allowed to regrow for another 13 weeks. Clove r growth at 7 weeks was in the following order: control < granite < KC l. The growth of ryegrass after 7 weeks was not significantly affected by granite as compared to the control treatment. After another 13 wee ks, both species showed a significant growth response to granite appli cation for two of the three soils studied. For both species and all th ree soils K concentrations in the plant tissue were systematically and significantly higher for KCl relative to granite and for granite rela tive to control treatment. Minor dissolution of granite occurred durin g the short duration of the experiment as indicated by changes in soil exchangeable K in uncropped pots (about 1-2% of K applied) and result ed in the increased K concentration in plants and the growth response of subterranean clover after 7 and 20 weeks and ryegrass after 20 week s of cropping. The possible use of granite powder as a slow-release K fertilizer is discussed.